1. Field of the Invention
Embodiments of the present invention relate generally to image processing systems, image sensor circuits, pixel circuits, image capturing methods, and image processing methods and, in specific embodiments, to an image sensor circuit including a pixel array and one or more circuits for controlling pixel circuits of the pixel array.
2. Related Art
Image sensor circuits are widely used to obtain images of physical scenes and objects. In many instances, image sensor circuits are employed to obtain images that are looked at and viewed by humans. In other instances, image sensor circuits are employed to obtain images that are used in machine vision and other automated pattern recognition processes. Related art image sensor circuits that place an emphasis on realistic depictions of scenes for human viewing may create some problems when used for pattern recognition applications.
Image sensor circuits typically include a pixel array with a plurality of pixel circuits arranged in rows and columns. Each of the pixel circuits typically includes a light sensitive element, such as a photodiode or the like, for sampling light intensity of a corresponding portion of a scene being imaged. During image capture, an accumulation of charge from the light sensitive elements in the pixel circuits of the pixel array is typically controlled in accordance with preset time periods specified for a shutter operation. Two types of shutter operations that are used in various related art image sensor circuits are: (i) a global shutter operation; and (ii) a rolling shutter operation.
In a typical global shutter operation, all pixel circuits in a pixel array are reset and then exposed simultaneously for a specified period of time to capture an image. With such global shutters, all pixel circuits in the pixel array start integrating or accumulating charge from light at a same first time point, and then stop accumulating charge at a same second time point. Thus, with such global shutters, all of the pixel circuits in the pixel array have a same integration time during which charge is accumulated from light for a scene being imaged.
In a typical rolling shutter operation, all pixel circuits in a same row of a pixel array are reset and then exposed simultaneously for a specified period of time. With such rolling shutters, all pixel circuits in a same row of the pixel array start accumulating charge from light at a same first time point, and then stop accumulating charge at a same second time point. Once a row of pixel circuits has been exposed for a specified integration time period in such a rolling shutter operation, the process continues to a next row in the pixel array, where all pixel circuits in the next row are then exposed simultaneously for the specified integration time period. The process then continues row-by-row through the pixel array until all of the rows of pixel circuits have been exposed for the specified integration time period to capture an image.
The global shutter and rolling shutter operations seek to preserve a relative relationship between points of light intensity in a scene, such that if one point in a physical scene is brighter than another point, then the same will be true in the captured image of the physical scene to the extent that the pixel circuits are not completely saturated. This is desirable when the captured images are for human viewing, because the captured images would be captured with the intent to maintain a realistic appearance of the physical scene. However, attempting to preserve a relative relationship between points of light intensity in a scene may lead to problems when capturing an image of a high dynamic range scene for pattern recognition purposes, because a variation in light intensity in the physical scene may exceed a dynamic range of the pixel circuits.
For example, consider a physical scene with a bright center portion and dark edges, such as when looking outside from inside of a dark tunnel on a bright afternoon. In such a case, if an integration time for accumulating charge in a global shutter or rolling shutter operation is set to be a long time so as to accumulate a sufficient amount of charge for the dark areas, then pixel circuits accumulating charge for the bright areas may saturate with charge. Such a saturation of charge may lead to an inability to see objects in the bright areas of the image. On the other hand, if the integration time for accumulating charge in such a case with a global shutter or rolling shutter operation is set to be a short time so as not to saturate the pixel circuits accumulating charge for the bright areas, then pixel circuits accumulating charge for the dark areas may not accumulate enough charge to allow for seeing objects in the dark areas.
The problem of accumulating too much charge or accumulating too little charge as described above can be very serious in the context of automated pattern recognition, because it is difficult and often impossible to recognize an object that cannot be seen in an image. For instance, in the example provided above, if captured images were being used to automatically control a car that is driving in the tunnel, then having a saturation of an image at the exit area of the tunnel may prevent the ability to recognize objects at the exit of the tunnel, which would adversely impact an ability to avoid such objects with the car. Thus, the global shutter and rolling shutter operations may lead to problems in cases of capturing images of high dynamic range scenes that have large differences in light intensity between different areas of the scenes.
FIG. 1 illustrates a block diagram of a prior art image sensor circuit 100. The image sensor circuit 100 includes a pixel array 101, an analog-to-digital converter (ADC) block 102, a digital image processor 103, a row addressing circuit 104, a control processor 105, and an image memory buffer 106. The pixel array 101 includes a plurality of pixel circuits 112 that are arranged in rows and columns. Each pixel circuit 112 includes a light sensitive element, such as a photodiode or the like, for sampling light intensity of a corresponding portion of a scene being imaged, and each pixel circuit 112 is configured to produce an analog pixel signal based on the sampled light intensity.
The pixel array 101 includes row control lines 1071, 1072, . . . , 107n, which may each include a plurality of control lines (not shown in FIG. 1), and the pixel array 101 also includes analog output lines 1081, 1082, . . . , 108m. The row addressing circuit 104 supplies control signals to the pixel circuits 112 in the pixel array 101 over the row control lines 1071, 1072, . . . , 107n to control an operation of the pixel circuits 112. Pixel circuits 112 that are in a same row of the pixel array 101, such as an ith row of the pixel array 101, share common row control signals over a common row control line 107i from the row addressing circuit 104. Pixel circuits 112 that are in a same column of the pixel array 101, such as a jth column of the pixel array 101, share a common analog output line 108j to provide output. The row addressing circuit 104 controls the pixel circuits 112 to perform processing row by row for a rolling shutter operation.
The analog pixel signals output from the pixel array 101 over the analog output lines 1081, 1082, . . . , 108m are input to the ADC block 102. The ADC block 102 typically includes one column ADC circuit 114 for each column of pixel circuits 112 in the pixel array 101. The column ADC circuits 114 are configured to convert analog pixel signals received from the pixel array 101 over respective ones of the analog output lines 1081, 1082, . . . , 108m into digital signals that are output on corresponding digital output lines 1091, 1092, . . . , 109m. The control processor 105 is configured to control an operation of the ADC block 102, and is also configured to control an operation of the row addressing circuit 104. The digital pixel signals output on the digital output lines 1091, 1092, . . . , 109m from the ADC block 102 are input to the digital image processor 103. The digital image processor 103, in cooperation with the image memory buffer 106 and the control processor 105, processes the input digital pixel signals to generate digital output signals on an output line 110.
FIG. 2 illustrates a prior art design for the pixel circuit 112. The pixel circuit 112 includes a photodiode 121, a transfer gate transistor 122, a sense node 131, a reset transistor 124, a drive transistor 125, and a read select transistor 126. The transfer gate transistor 122, the reset transistor 124, the drive transistor 125, and the read select transistor 126 each comprise an N-channel metal oxide semiconductor (NMOS) field effect transistor. A generic one of the row control lines 1071, 1072, . . . , 107n (refer to FIG. 1) is shown in FIG. 2 as a row control line 107, and a generic one of the analog output lines 1081, 1082, . . . , 108m (refer to FIG. 1) is shown in FIG. 2 as an analog output line 108. The row control line 107 includes a row readout signal line 127, a transfer signal line 129, and a reset signal line 130. The pixel circuit 112 receives input signals on the row readout signal line 127, the transfer signal line 129, and the reset signal line 130. The pixel circuit 112 provides output signals on the analog output line 108.
As illustrated in FIG. 2, the photodiode 121 is connected between ground 133 and a first terminal of the transfer gate transistor 122. A second terminal of the transfer gate transistor 122 is connected to the sense node 131, and a gate of the transfer gate transistor 122 is connected to the transfer signal line 129. A first terminal of the reset transistor 124 is connected to a voltage source 132, a second terminal of the reset transistor 124 is connected to the sense node 131, and a gate of the reset transistor 124 is connected to the reset signal line 130. A first terminal of the drive transistor 125 is connected to the voltage source 132, a second terminal of the drive transistor 125 is connected to a first terminal of the read select transistor 126, and a gate of the drive transistor 125 is connected to the sense node 131. A second terminal of the read select transistor 126 is connected to the analog output line 108, and a gate of the read select transistor 126 is connected to the row readout signal line 127.
FIG. 3 illustrates a prior art design for the column ADC circuit 114. The column ADC circuit 114 includes a source transistor 140, a double sampling amplifier 142, and an analog-to-digital converter (ADC) circuit 144. The double sampling amplifier 142 is controlled by control signals provided from the control processor 105 (refer to FIG. 1), which are received by the double sampling amplifier 142 over an amplifier control signal line 146. The ADC circuit 144 is controlled by control signals provided from the control processor 105 (refer to FIG. 1), which are received by the ADC circuit 144 over a converter control signal line 148. A generic one of the analog output lines 1081, 1082, . . . , 108m (refer to FIG. 1) is shown in FIG. 3 as the analog output line 108, and a generic one of the digital output lines 1091, 1092, . . . , 109m (refer to FIG. 1) is shown in FIG. 3 as a digital output line 109. A first terminal of the source transistor 140 is connected to the analog output line 108, and a second terminal of the source transistor 140 is connected to ground 133. An input of the double sampling amplifier 142 is connected to the analog output line 108, and an output of the double sampling amplifier 142 is connected to an input of the ADC circuit 144. An output of the ADC circuit 144 is connected to the digital output line 109.
FIG. 4 illustrates the prior art image sensor circuit 100 of FIG. 1, in which the pixel circuit 112 of FIG. 2 and the column ADC circuit 114 of FIG. 3 are depicted. An operation of the image sensor circuit 100 is now described with reference to FIGS. 1, 2, 3, and 4. When an image capture operation is initiated, photodiode 121 is reset by providing both a HIGH signal on the transfer signal line 129 to turn on the transfer gate transistor 122 and a HIGH signal on the reset signal line 130 to turn on the reset transistor 124. A LOW signal is then provided on the reset signal line 130 to turn off the reset transistor 124, while the transfer gate transistor 122 remains on to allow charge generated in the photodiode 121 to accumulate in the sense node 131. At an end of an exposure time interval, a LOW signal is provided on the transfer signal line 129 to turn off the transfer gate transistor 122.
Once the transfer gate transistor 122 has been turned off or closed, a HIGH signal is provided on the row readout signal line 127 to turn on the read select transistor 126, and the double sampling amplifier 142 samples a pixel circuit output voltage on the analog output line 108. Then, a LOW signal is provided on the row readout signal line 127 to turn off the read select transistor 126, and a HIGH signal is provided on both the reset signal line 130 and the transfer signal line 129 to turn on the reset transistor 124 and the transfer gate transistor 122, so as to reset the sense node 131. When the sense node 131 is in a reset state, a HIGH signal is provided on the row readout signal line 127 to turn on the read select transistor 126, and the double sampling amplifier 142 samples a pixel circuit reset voltage on the analog output line 108. The double sampling amplifier 142 then computes a difference between the pixel circuit output voltage and the pixel circuit reset voltage to arrive at a corrected pixel circuit output voltage. The corrected pixel circuit output voltage is provided from the double sampling amplifier 142 to the ADC circuit 144, and the ADC circuit 144 converts the corrected pixel circuit output voltage to a digital signal and provides the digital signal to the digital image processor 103.
In the image sensor circuit 100, all pixel circuits 112 in a given row of the pixel array 101 accumulate charge for an equal amount of time. Thus, the image sensor circuit 100 has problems as discussed above when capturing an image of a high dynamic range scene for pattern recognition purposes, because a variation in light intensity in the physical scene may exceed a dynamic range of the pixel circuits 112. Such problems may prevent objects or patterns from being recognized in images captured by the image sensor circuit 100.